The concept of Orthogonal Sub-channels (OSCs) and how they can be used to increase the voice capacity of GSM EDGE Radio Access Network (GERAN) cells has been disclosed by others (GSM is Global System for Mobile; EDGE is Enhanced Data rates for Global Evolution). In an OSC scheme, the base station (BS) uses Quadrature Phase-Shift Keying (QPSK) modulation for downlink (DL), which multiplexes voice data from two users. The multiplexing is done such that legacy Mobile Stations (MSs) using Gaussian Minimum Shift Keying (GMSK) can receive their respective data.
As an example, FIG. 1 shows a QPSK constellation chosen as a subset of an Eight Phase Shift Keying (8PSK) constellation. The most significant bit (MSB) and least significant bit (LSB) define two “orthogonal” sub-channels I and Q, wherein the bits are denoted as (OSC0, OSC1). Each sub-channel carries voice signals of two users in the DL direction. GMSK-only capable MSs are able to detect the individual sub-channels.
These prior OSC proposals also provide that downlink power control may use conditions of the weakest link as criteria. For example, in such an approach, if the weaker orthogonal sub-channel is I, the power control would be adjusted such that both sub-channels I and Q are equally increased until the sub-channel I attains the minimum acceptable power level. This approach would have the advantage of maintaining the shape of the QPSK constellation as circular, keeping all four constellation points equidistant, which provides maximum separation for best receiver decoding results. The disadvantage to this approach is that more power is used than is necessary on the sub-channel that is not the weakest link, (i.e., sub-channel Q in this example). Consequently, the interference between the sub-channels will increase.